It is often necessary when testing materials or operating a device to measure or monitor a material property known as “strain.” Both tensile and compressive strain may be of interest, depending upon the material and the operating conditions to which the material will be subjected. Strain is defined as the differential elongation of a body under load divided by the length of the body when it is not being loaded.
The differential elongation of many bodies is very small, on the order of a few thousandths or ten-thousandths of an inch, making direct measurement of the strain difficult. It is common practice to affix or attach a strain sensor to a body to measure strain in the body.
One type of strain sensor, known as a magnetostrictive strain sensor, includes magnetic elements that form a magnetic flux path, and a magnetized element for establishing a magnetic flux field within the magnetic flux path. One of the magnetic elements of the sensor, hereinafter referred to as the magnetostrictive element, is affixed to the body in such a manner that as the body elongates or is compressed under load, the magnetostrictive element is also elongated or compressed an equal distance. As the length of the magnetostrictive element is changed, its magnetic permeability is also changed, which in turn causes a change in the magnetic field in the flux path. By sensing the change in the magnetic flux, the strain in the magnetostrictive element and the body to which it is affixed can be determined.
The change in magnetic flux is often detected by measuring the change in electric current flow, induced by the magnetic flux within the sensor flux path, in a coil of wire wrapped externally around a portion of the magnetic flux path. The change in current flow is measured and compared to a calibration table to determine the strain in the body.
Although many magnetic materials, including steel and Nickel, can be utilized as the magnetostrictive element in a strain sensor, certain materials exhibit greater changes in magnetic permeability when subjected to a load, i.e. stronger magnetostrictive performance, than other materials. One material offering particular advantages when used as the working element in magnetostrictive strain sensors is known as TERFENOL-D. TERFENOL-D is an alloy of terbium, iron, and Dysprosium-D, developed by the United States Navy, and sold under the trade name TERFENOL-D® by ETREMA Products, INC., of Ames, Iowa, U.S. The change in magnetic permeability of TERFENOL-D under load is thousands of times greater than steel, and is thus more readily detectable. TERFENOL-D is also hundreds of times stronger than Nickel, allowing a more robust sensing device to be provided.
While magnetostrictive strain sensors of the type described above are acceptable for many applications, they do have some undesirable drawbacks. The external coils in such sensors cause the sensor to be bulky, and may not allow the sensor to fit into the space available for the strain sensor in some applications. Calibration of the sensor can also be difficult.
What is needed, therefore, is an improved magnetostrictive strain sensor.